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It depends on your configuration. If you are delivering mail to people who have shell accounts, you must specify the username, and not the email address (the domain is implied, and dovecot won't know what to do with it).

If you are delivering mail to virtual mailbox users, and the virtual mailboxes are mapped using the full email address, you need to give the email address there. It all depends on how you've mapped it.

I have a setup where some users have their unix username as their login (as they have shell accounts) and others, on some hosted domains, use their email address. These configuration directives make it recognize unix usernames: